Depending on its missions, a spacecraft, more particularly a satellite such as an observation satellite, must be able to be oriented and stabilized in accurate particular directions.
This general problem is particularly critical for observation satellites for which the optical line of sight of an observation instrument must be able to quickly tilt depending on the directions of the centers of interest.
Today, for this type of attitude and stabilization control, two main families of actuators are used.
A first family corresponds to reaction wheels.
A reaction wheel generally includes a disc commanded and controlled in rotation about a fixed axis in a reference system linked to the spacecraft aboard which it is installed.
When the disc, coupled to the driving motor, is accelerated or slowed, the variation of the angular momentum of the disc is transmitted to the spacecraft which then rotates about the axis corresponding to the variation of the angular momentum of the disc.
By coupling three or more reaction wheels, the axes of rotation of which have different directions in the reference system of the spacecraft, it is possible to orientate the spacecraft in every direction.
Such a known device is renowned for its robustness, more particularly because of the small numbers of movable parts. In addition, it generates torques along a fixed direction with respect to the spacecraft, which simplifies the attitude control laws. However, it has the drawback of consuming energy each time the disc rotation speeds must be accelerated to create angular momentum, and the energy is generally dissipated when the disc rotation speed is decreased to destroy angular momentum. Furthermore, the obtainable maximum torque is limited to at most a few Nm for space applications because its efficiency is in the wheel/satellite inertia ratio and because the required power increases with the wheel speed.
Another family corresponds to gyroscopic actuators.
According to the general principle, a gyroscopic actuator includes a constant speed rotation momentum wheel rotating about an axis, the orientation of which can be modified with respect to a reference system of the spacecraft.
Therefore, the momentum wheel is carried by a gimbal device controlled in position, with the wheel rotation speed being kept constant.
When the orientation of the wheel axis of rotation, i.e. the orientation of the corresponding angular momentum, is modified in a reference system of the spacecraft through an action of the gimbal device, a torque is generated which is transmitted to the spacecraft through the actuator's supports.
By coupling several gyroscopic actuators, it is possible to orient the spacecraft in every direction.
This type of actuator is advantageous since the creation of the torque is obtained through the transfer of angular momentum without loss of the wheel mechanical energy (except for frictions). For a given electric power, the gyroscopic actuator can obtain much more important torques (typically a hundred times more important) than the reaction wheel.
However, the systems using gyroscopic actuators are relatively complex because of the complexity of the actuators themselves which are provided with gimbal devices and because the gyroscopic torques generated by this type of actuator are rotating in the satellite axis, which requires complex guidance and control laws for the implementation thereof.
Such attitude control devices with gyroscopic actuators also have the drawback of having singular points for which the attitude control cannot be normally provided, which leads to oversize the system of actuators, for example using four actuators instead of three for the spacecraft attitude control along the three axes thereof.